Git — updating branches when there is an error in the kernel project

Let's say that you write 2 of the project (the directory "project1", "project2") sharing a common core (the folder "core") — we have two branches in git that don't know about each other, nothing except the very first commit in the kernel.
During the development of one of them we find a big mistake (not one line and not single file) in the kernel.
How to update the second project?
October 8th 19 at 02:28
1 answer
October 8th 19 at 02:30
As a variant — to make the core into a separate repository in project1 and project2 to plug in as submodule

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